The fabrication of integrated circuits is typically accomplished using a lithographic process in which a stepper or scanner machine is used to print integrated circuits on silicon wafers. In state of the art fabrication plants, the light sources for this lithographic process are excimer lasers, most of which are narrow band KrF excimer lasers operating at a wavelength of about 248 nm. In the future, greater resolution, than is possible with the 248 nm wavelength light will be provided by an industry shift to ArF excimer lasers operating at about 193 nm and F2 excimer lasers operating at about 152 nm.
Since the fabrication lines including the stepper/scanner machines and their associated laser light sources are very expensive and the integrated circuits they produce are very valuable, the integrated circuit fabrication lines typically operate almost continuously, xe2x80x9cround the clockxe2x80x9d, 24 hours a day, 7 days per week 365 days per year with the minimum possible down time for maintenance. Therefore, great efforts go into building and servicing the fabrication line equipment, including the excimer lasers, to minimize down time, especially unscheduled down time. As a result, lithography excimer lasers are expected to have xe2x80x9cdown timexe2x80x9d of much less than 1%.
In addition, the quality of the integrated circuits produced on these fabrication lines is to a large extent dependent on the quality of the laser beam produced by the laser
The beam as indicated above is typically line narrowed and the energy of each pulse is carefully controlled. Beam quality specification parameters of centerline wavelength, bandwidth and pulse energy are typically monitored for each pulse which in state of the art lithography lasers operate at pulse rates of between 1,000 and 2,500 pulses per second. Beam specifications for a typical KrF excimer laser might be:
These specifications are examples of the type of quality standards which are applied to determine if a laser""s performance passes an acceptance test prior to shipment from the laser fabrication plant.
During operation of the integrated circuit fabrication line, energy, center wavelength, and bandwidth are monitored and energy and centerline wavelength are controlled with automatic computer based feedback controls. Various methods are used by the operators of the fabrication lines to control the quality of the laser beam and to make decisions as to when adjustments, maintenance or equipment replacement is necessary. These decisions are often difficult to make when beam quality deteriorates because shutting down a production line for repairs usually involves production losses which increase unit production costs of the integrated circuits. On the other hand, continuing to produce with less than ideal beam quality results in reduced quality.
U.S. Pat. No. 5,646,954 (incorporated herein by reference) describes a prior art maintenance strategy control system and monitoring method for improving performance reliability of excimer laser lithography light sources. This system uses microprocessors to monitor laser pulses and to predict based on usage values when maintenance and equipment replacement should be scheduled. Lithography lasers are typically built in modular form so that an entire module is quickly replaced with a spare module whenever there is a failure within the module. The replaced module is then returned to the factory. The reusable parts in it are recycled into newly manufactured modules. Examples of such modules include chamber modules comprising the laser chamber and associated components, stabilization modules comprising a wavemeter for stabilizing the wavelength and pulse energy of the laser beam and a line narrowing module (LNM) for narrowing the bandwidth of the laser beam and controlling the wavelength of the beam.
Control of laser beam quality is very important to maintaining high quality integrated circuit production. State of the art lithography lasers such as KrF excimer lasers comprise three information control-data ports:
(1) a stepper/scanner port through which the stepper/scanner computer controller issues firing commands to the laser computer.
(2) a serial port for a laser control device in the shape of a paddle and called a paddle through which laser operators send serial commands to the laser to control laser parameters such as target wavelength, pulse energy or makes adjustments to parameters such as gas mixtures.
(3) An RS-232 diagnostic port used by field engineers to collect parametric data from the laser computer.
A typical current process for collecting and processing data from lithography lasers by the laser manufacturer involve the following steps:
a) Field engineers from the laser manufacturer physically download the data from the lithography lasers every week
b) This data is stored in the form of ASCll flat files and then sent by E-mail to laser manufacturer for further processing.
c) This E-mail based program has been in use for 3 years, which parses data files of E-mails and saves them in a server.
d) This data is viewed/massaged by experienced factory technical support personnel for discrepancy.
e) Ad-hoc queries are generated by user(s) based on the data in SQL Server Database for historical analysis.
f) The entire process is time sensitive (e.g. certain queries take between 7 to 8 minutes for processing).
The following limitations exist since the current process is manual in nature:
a) Field engineers must be physically present in the clean room environment at the integrated circuit fabrication plant to download this data every week. Hence, this process cannot be done daily but has to be scheduled with the customer/field engineers. The data currently is being collected every week.
b) The data is not real time in any nature and by the time it is processed the data may be more than a week old.
c) There is no continuous status reporting.
d) Training of field engineers to download the data in proper format is necessary.
e) There are no automatic alerts to concerned personnel.
f) The entire process has a degree of uncertainty in operating these laser machines and represents substantial overhead cost to both the laser supplier and the fabrication plant.
What is needed is a better system for monitoring lithography lasers.
The present invention provides a system for a monitoring lithography lasers at integrated circuit fabrication plants. Each laser at each fabrication plant has associated with it a terminal server. With respect to each fabrication plant a central control server unit is in communication with each of the lasers through a local area network. Information from the lasers is collected by the central control server unit and the information is used to provide summary information which is made available in a web site format to interested parties having access authorization.
A principal function of the present invention is data acquistion. Monitors on the lasers record an enormous amount of data. For example, each laser pulse is monitored for pulse energy, wavelength, bandwidth and charging voltage. Since a state of the art laser typically operates at 2000 Hz at duty cycles of about 20 percent, just these parameters represent 1600 values collected each second and since operation is around the clock this data amounts to about 138 million values of primary laser data per day. In addition, the laser calculates other performance values from this primary data. The calculated values include standard deviation values of both wavelength and pulse energy for small groups of data. The laser also calculates a dose variation value for designated groups of pulses referred to as xe2x80x9cwindowsxe2x80x9d of pulses. In addition, other laser parameters are monitored very frequently and may be recorded as often as desired. These other parameters include various temperature values, laser gas pressure and fan speed.
Typically the lithography lasers are operated in bursts mode in which short bursts of pulses (such as 200 pulses) are produced (during which time a single dye spot on a wafer is illuminated) followed by an idle time of a fraction of a second during which time the stepper or scanner moves to a different dye spot. After all the dye spots on a wafer are illuminated there is a longer idle time of a few seconds during which time a new wafer is moved into place. The laser monitors this pattern and the present system is capable of documenting each and every pulse of every burst along with the idle times. In addition any desired summaries, compilations, reports, tables which are aggregates or comparisons of the data may be calculated and stored and made available on an almost real-time basis.
Charts could include:
1) For each laser up-time (or down time) on a monthly basis
2) For each laser duty cycle during up-times
3) For each chamber integrated pulse count since last chamber replacement
4) For each module (such as LNP, Power supply module, commutator module, compression lead module, gas module) integrated pulse count (or days) since last module replacement
5) For each laser at specified time intervals: wavelength monitor, wavelength sigma, average bandwidth, dose variation, energy sigma and energy variation.